Layer-by-Layer Surface Modification of Functional Nanoparticles for

Oct 28, 2010 - In order to prepare SiO2 nanoparticles that are dispersible in various organic solvents, an anionic surfactant 1, which branches into a...
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Layer-by-Layer Surface Modification of Functional Nanoparticles for Dispersion in Organic Solvents Motoyuki Iijima* and Hidehiro Kamiya Institute of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan Received August 2, 2010. Revised Manuscript Received October 8, 2010 In order to prepare SiO2 nanoparticles that are dispersible in various organic solvents, an anionic surfactant 1, which branches into a hydrophobic chain and a hydrophilic chain, was adsorbed on to SiO2 nanoparticles through a layer-by-layer surface modification route using polyethyleneimine (PEI). First, the relationship among the additive content of PEI, adsorbed content of PEI, and the redispersion stability of the SiO2 nanoparticles in water was investigated. While almost the entire PEI was adsorbed when the additive PEI content was lower than 67 mg/g of SiO2, the adsorbed content of PEI became saturated when the additive content was increased above 90 mg/g of SiO2. SiO2 nanoparticles that were saturated with PEI could be redispersed into water at sizes close to their primary particle size without the large-scale formation of aggregates. Next, the anionic surfactant 1 was adsorbed on the SiO2 nanoparticles by using a SiO2 aqueous suspension saturated with adsorbed PEI. It was found that the adsorbed content of 1 increased almost linearly as the additive content was increased when the additive condition was below 1400 mg/g of SiO2. Furthermore, SiO2 nanoparticles adsorbed with 80 mg/g of SiO2 of PEI and 810 mg/g of SiO2 of 1 could be dispersed into various organic solvents with different polarities. This layer-by-layer modification technique can also be applied to Ag nanoparticles in order to prepare Ag nanoparticles that can be dispersed in various organic solvents.

1. Introduction The use of nanoparticles has became indispensable in industrial applications because of their unique size-dependent properties such as their electrical, magnetic, mechanical, optical, and chemical properties, which differ considerably from those of the corresponding bulk materials.1-4 Many studies have been conducted to enable the use these functional nanoparticles in the fabrication of nanostructured devices such as polymer composites, ceramic composites, composited particulates, and assembled structures because of their wide range of important applications.5-10 In order to design nanostructured devices with good properties, techniques that can completely disperse nanoparticles into a particular solvent must be developed. Furthermore, the ability to control the dispersion stability of nanoparticles during the processing procedures is quite essential. Until today, many researchers have reported the synthesis of nanoparticles that can be completely redispersed in organic solvents using various methods such as the reverse micelle method,11,12 *Corresponding author. E-mail: [email protected]. Phone/Fax: þ81-42-388-7068.

(1) Alivisatos, A. P. Science 1996, 271, 933. (2) Murphy, C. J.; Jana, N. R. Adv. Mater. 2002, 14, 80. (3) Kimberly, D.; Dhanasekaran, T.; Zhang, Z.; Meisel, D. J. Am. Chem. Soc. 2002, 124, 2312. (4) Ozin, G. A. Adv. Mater. 1992, 4, 612. (5) Zhang, H.; Cui, Z.; Wang, Y.; Zhang, K.; Ji, X.; Lu, C.; Yang, B.; Gao, M. Adv. Mater. 2003, 15, 777. (6) Althues, H.; Palkovits, R.; Rumplecker, A.; Simon, P.; Sigle, W.; Bredol, M.; Kynast, U.; Kaskel, S. Chem. Mater. 2006, 18, 1068. (7) Zhang, X. F.; Harley, G.; De Jonghe, L. C. Nano Lett. 2005, 5, 1035. (8) Limthongkul, P.; Wang, H.; Chiang, Y.-M. Chem. Mater. 2001, 13, 2397. (9) Caruso, F.; Spasova, M.; Salgueiri~no-Maceira, V.; Liz-Marzan, L. M. Adv. Mater. 2001, 1090. (10) Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O’Brien, S.; Murray, C. B. Nature 2006, 439, 55. (11) Capek, I. Adv. Colloid Interface Sci. 2004, 110, 49. (12) Lopez-Quintela, M. A. Curr. Opin. Colloid Interface Sci. 2003, 8, 137. (13) O’Brien, S.; Brus, L.; Murray, C. B. J. Am. Chem. Soc. 2001, 123, 12085.

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sol-gel reactions of metal alkoxides involving capping agents,13,14 nonhydrolytic sol-gel reactions of metal halides with capping agents,15-17 thermal decomposition of metal complexes,18,19 and polyol methods.20,21 By using these methods, various oxides, sulfides, and metal nanoparticles can be redispersed in adequate organic solvents. However, a major problem that still exists is that these nanoparticles can only be dispersed into limited solvents related to their capping agents. For example, nanoparticles capped with fatty acids can only be dispersed into low polar solvents, whereas those capped with polyols can only be dispersed into highly polar solvents such as water and alcohols. This limited dispersibility of nanoparticles causes considerable problems in the fabrication of nanostructured devices. For instance, during the fabrication of polymer nanocomposites, nanoparticle fillers are incorporated into the polymer matrix by the polymerization of nanoparticle/monomer suspensions or by the evaporation of the solvent in the nanoparticle/solvent/polymer mixtures. Even though homogeneous nanoparticle/monomer suspensions and nanoparticle/solvent/polymer mixtures have been successfully prepared, these nanoparticles form severe aggregates during the polymerization process or the solvent evaporation process due to the polarity changes of the matrixes. (14) Cozzoli, P. D.; Kornowski, A.; Weller, H. J. Am. Chem. Soc. 2003, 125, 14539. (15) Trentler, T. J.; Denler, T. E.; Bertone, J. F.; Agrawal, A.; Colvin, V. K. J. Am. Chem. Soc. 1999, 121, 1613. (16) Jun, Y.; Casula, M. F.; Sim, J.-H.; Kim, S. Y.; Cheon, J.; Alivisatos, A. P. J. Am. Chem. Soc. 2003, 125, 15981. (17) Niederberger, M.; Bartl, M. H.; Stucky, G. D. J. Am. Chem. Soc. 2002, 124, 13642. (18) Rockenberger, J; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 1999, 121, 11595. (19) Park, J.; An, K.; Hwang, Y.; Park, J.-G.; Noh, H.-J.; Kim, J.-Y.; Park, J.-H.; Hwang, N.-M.; Hyeon, T. Nat. Mater. 2004, 3, 891. (20) Feldmann, C. Adv. Func. Mater. 2003, 13, 101. (21) Zhao, Y.; Zhang, Y.; Zhu, H.; Hadjipanayis, G. C.; Xiao, J. Q. J. Am. Chem. Soc. 2004, 126, 6874.

Published on Web 10/28/2010

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2.2. Synthesis and Surface Modification of SiO2 Nanoparticles. First, SiO2 nanoparticles, ca. 30 nm in diameter, were Figure 1. Anionic surfactant 1 branched into hydrophilic group containing polymerizable group and hydrophobic group near the headgroup.22

To overcome this problem, we have previously reported the synthesis of an anionic surfactant that branches into a hydrophobic alkyl chain and a hydrophilic polyethylene glycol chain containing a polymerizable group, near the headgroup (1; Figure 1).22 The concept of having the hydrophobic chain, hydrophilic chain, and the polymerizable groups was to increase the affinity of nanoparticles modified with this surfactant with low polar solvents, polar solvents, and polymerizable polymers, respectively. By applying this anionic surfactant to TiO2 nanoparticles, we were able to prepare TiO2 nanoparticles that are dispersible in various types of organic solvents, including toluene, tetrahydrofuran, methylmethacrylate, ethanol, acetonitrile, and methylethylketone. Furthermore, TiO2 nanoparticles capped with 1 could also be dispersed not only into solvents but also into different types of polymers such as epoxy and PMMA without strong aggregation, resulting in the formation of transparent TiO2/polymer nanocomposites. Although the developed surfactant 1 can play an important role for the preparation of functional nanoparticles dispersible into various solvents/polymers, 1 can only be effectively adsorbed onto limited materials such as TiO2. In this paper, we report a layer-by-layer surface modification route by which SiO2 nanoparticles can adsorb the anionic surfactant 1 and can be dispersed into various types of organic solvents. SiO2 nanoparticles were selected as a model particle because the anionic surfactant 1 cannot be directly adsorbed on it. Polyethyleneimine (PEI), a cationic polymeric dispersant, was first adsorbed on SiO2 nanoparticles, and then the anionic surfactant 1 was adsorbed. Although several previous studies have reported on the modification of the surface of particles with PEI for their further use in layerby-layer surface modification,23-25 the effect of the addition of PEI on the redispersion properties of modified SiO2 nanoparticles in aqueous media has not yet been clearly understood. Therefore, the relationships among the additive content of PEI, adsorbed content of PEI on SiO2 nanoparticles, and redispersion stability of the nanoparticles in water is first investigated. Then, the adsorption properties of 1 on PEI-modified SiO2 nanoparticles in deionized water and their stability in various organic solvents is studied. Furthermore, this layer-by-layer surface modification process is also applied to Ag nanoparticles to show the applicability of this process to various materials.

2. Experimental Section 2.1. Materials. Tetraethyl orthosilicate (TEOS, >95%), ethanol (95%), ammonia aqueous solution (28 wt %), polyethyleneimine (PEI, average molecular weight 1800), toluene (99.5%), L-ascorbic acid (99.6%), and oleylamine were purchased from Wako Pure Chemical Industry Ltd., Japan. Silver nitrate (99.8%) was purchased from Kanto Chemical Co., Inc. All materials were used without further purification. Anionic surfactant 1, which contains hydrophilic and hydrophobic chains, was prepared as previously reported.22 (22) Iijima, M.; Kobayakawa, M.; Yamazaki, M.; Ohta, Y.; Kamiya, H. J. Am. Chem. Soc. 2009, 131, 16342. (23) Sukhorrukov, G. B.; Donath, E.; Davis, S. A.; Lichtenfeld, H.; Caruso, F.; Popov, P. I.; Mohwald, M. F. Polym. Adv. Technol. 1998, 9, 759. (24) Gittins, D. I.; Caruso, F. J. Phys. Chem. B 2001, 105, 6846. (25) Bringley, J. F.; Wunder, A.; Howe, A. M.; Wesley, R. D.; Qiao, T. A.; Liebert, N. B.; Kelley, B.; Minter, J.; Antalek, B.; Hewitt, J. M. Langmuir 2006, 22, 4198.

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prepared by the St€ ober method in a similar manner according to the previous report.26 Briefly, 50 g of TEOS was rapidly added into a mixture of 3.8 g of ammonia solution, 60 g of water, and 590 g of ethanol under stirring. The mixture was stirred at 25 °C for 3 days. Then, the prepared SiO2 nanoparticles were modified with PEI by the following method. A total of 20 g of the prepared SiO2 suspension was mixed with various amount of 20 wt % PEI aqueous solution with stirring. The additive content of PEI was controlled between 20 mg/g of SiO2 and 225 mg/g of SiO2. After stirring for 30 min, the SiO2 nanoparticles, which became weakly flocculated, were collected by centrifugation (30,000g, 10 min), rinsed with deionized water, and then redispersed into 80 g of deionized water with sonication. This SiO2 suspension with the adsorbed PEI was further used for the adsorption test of the anionic surfactant 1 and for characterizing the adsorbed content of PEI. For the adsorption of anionic surfactant 1, SiO2 suspensions were treated with 225 mg/g of SiO2 of PEI. Various amounts of anionic surfactant 1 aqueous solution (40 wt %) were added to the SiO2/PEI suspension prepared as described above with stirring. The additive content of the anionic surfactant 1 was controlled between 270 mg/g of SiO2 and 1900 mg/g of SiO2. After stirring for 60 min, the flocculated SiO2 nanoparticles were collected by centrifugation (30,000g, 5 min), rinsed with water, and dried under vacuum at 80 °C.

2.3. Synthesis and Surface Modification of Ag Nanoparticles. Ag nanoparticles capped with oleylamine were prepared in

a manner similar to the previous report.27 Briefly, 170 mg of silver nitrate was dissolved into a mixture of 10 mL of toluene and 8.1 g of oleylamine by ultrasonication. Then, 350 mg of L-ascorbic acid was added into the mixture with stirring, and the solution was stirred for 2 h at room temperature. After the mixing procedure, the synthesized oleylamine-capped Ag nanoparticles were weakly flocculated by the addition of 60 mL of ethanol and separated by centrifugation (30,000g, 10 min.). The separated Ag nanoparticles were washed with 30 mL of ethanol and redispersed into 20 mL of toluene. Next, the capping agent was exchanged with PEI. Here, 20 mL of the prepared Ag suspension in toluene was mixed with 1.0 mL of 50 wt % PEI solution and then gently mixed for 2 h. After 2 h, the sedimented, brown oily product was collected and washed with 20 mL of acetone, twice. The obtained product was then dispersed into 20 mL of deionized water and 200 μL of 40 wt % anionic surfactant 1 solution was added. After mixing for 30 min, the sediments, which appeared after the anionic surfactant addition was collected, were washed with 20 mL of deionized water and dried under vacuum. 2.4. Characterizations. The adsorbed content of PEI and anionic surfactant 1 was characterized by TG-DTA analysis performed on a RIGAKU Thermo Plus EVO. Fourier transform infrared (FT-IR-ATR) and near-infrared (FT-nIR) spectra of these particles were obtained using Nicolet Nexus 470. The ζ-potential of raw SiO2 nanoparticles and nanoparticles containing PEI were characterized using Zeta Potential Analyzer Model 502 (Nihon Rufuto Co. Ltd.), which is a microscope module ζ-potential measurement apparatus. The ability of the prepared particles to be dispersed into solvents was determined by dynamic light scattering (DLS) measurements performed on Malvern HPP5001. The measurements were performed after treating the suspension with ultrasonication (300W, Nihonseiki Kaisha Co., US-300T, Japan) for 1 min.

3. Results and Discussion 3.1. Surface Modification of St€ ober SiO2 Nanoparticles by PEI. Figure 2 shows the FT-nIR spectra of SiO2 nanoparticles (26) Kamiya, H.; Suzuki, H.; Kato, D.; Jimbo, G. J. Am. Ceram. Soc. 1993, 76, 54. (27) Kim, S. -G.; Haruga, N.; Iskandar, F.; Okuyama, K. Adv. Powder. Technol. 2009, 20, 94.

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Figure 4. Relationship among additive content of PEI, adsorbed content of PEI on SiO2 nanoparticles, and ζ-potential of PEImodified SiO2 nanoparticles measured at pH 8.

Figure 2. FT-nIR spectrum of SiO2 nanoparticles (a) before modification and (b) after PEI adsorption. The additive content of PEI was 22 mg/g of SiO2.

Figure 5. Effect of additive content of PEI on redispersion stability of PEI-modified SiO2 nanoparticles in deionized water.

Figure 3. TG curves of PEI-modified SiO2 nanoparticles. The additive content of PEI was (a) 22 mg/g of SiO2, (b) 67 mg/g of SiO2, (c) 150 mg/g of SiO2, and (d) 225 mg/g of SiO2.

before and after modification with PEI. From the nIR spectra, it was found that both of the peaks for the hydrogen-bonded Si-OH groups (4415 cm-1) and residual Si-OR (alkoxy) groups (4350 cm-1)28 were detected in the raw SiO2 nanoparticles, whereas the peak that corresponds to the residual Si-OR group disappeared in the spectra of SiO2 nanoparticles modified with PEI. It is expected that the adsorption of PEI, which possesses basic properties, on to SiO2 nanoparticles in aqueous media enhanced the hydrolysis of the unreacted residual Si-OR and Si-OH groups. Figure 3 shows several examples of TG curves of SiO2 nanoparticles treated with various amounts of PEI. In all samples, weight loss according to the evaporation of adsorbed water and decomposition of organic compounds can be observed near 100 °C and over 200 °C, respectively. Since it was found that these peaks correspond to residual alkoxy groups that disappeared after PEI adsorption, the decomposition of this organic compound is expected to be caused by the decomposition of the adsorbed PEI. In order to evaluate the relationship between the additive content of PEI and its adsorbed content, the adsorbed amount of PEI was calculated by subtracting the water content from the total weight loss and the results are plotted in Figure 4. Figure 4 shows that the adsorbed content of PEI increases as the additive content of PEI increases up to 67 mg/g of SiO2. At this (28) Orgaz, F.; Rawson, H. J. Non-Cryst. Sol. 1986, 82, 57.

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point, almost all added PEI was adsorbed on the surface of SiO2 nanoparticles. In contrast, when the additive content was increased to more than 90 mg/g of SiO2, the adsorbed content did not increase, but rather saturated the system. The saturated adsorption of PEI on SiO2 nanoparticles can also be confirmed from the changes in ζ-potential values. While the ζ-potential at pH 8 was negative for the SiO2 nanoparticles without PEI, that value gradually became positive when the adsorbed content of PEI increased due to the amine groups of PEI. While the ζ-potential value increased for SiO2 nanoparticles treated with 22-67 mg/g of SiO2 of PEI, it was saturated when the additive PEI content increased more than 90 mg/g of SiO2. In order to clarify the effect of the additive content of PEI on the redispersion stability of PEI-modified SiO2 nanoparticles in deionized water, the particle size distribution of SiO2 nanoparticles was analyzed by the DLS method (Figure 5). SiO2 nanoparticles were collected by centrifugation after PEI adsorption and then redispersed into deionized water to 0.5 wt %. In Figure 5, the filled symbols and the open symbols represents SiO2 nanoparticles with an unsaturated amount of PEI and a saturated amount of PEI, respectively. For SiO2 nanoparticles adsorbed with an unsaturated amount of PEI (67 mg/g of SiO2), the aggregated particle size was relatively large compared with those of raw SiO2 nanoparticles, and these particles could not be completely redispersed into deionized water. Furthermore, for the samples treated with lower PEI levels (below 67 mg/g of SiO2), the SiO2 nanoparticles rapidly formed large aggregates and sediments, so it was impossible to analyze the particle size by DLS methods. In contrast, the SiO2 nanoparticles adsorbed with a saturated concentration of PEI were able to be redispersed into deionized water and their particle size distributions were quite similar to those of DOI: 10.1021/la1030747

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Figure 6. PEI-adsorbed SiO2 suspension (a) before and after addition of (b) 600 mg/g of SiO2 and (c) 1900 mg/g of SiO2 of anionic surfactant 1. (d) Changes in transmittance value of SiO2 suspension before and after addition of anionic surfactant.

raw SiO2 nanoparticles. The saturated adsorption of PEI on the SiO2 nanoparticles caused effective steric repulsive forces between the particles to prevent severe aggregation and allowed PEIcoated SiO2 nanoparticles to be dispersed into deionized water. 3.2. Adsorption of Anionic Surfactant 1 on PEI-Coated SiO2 Nanoparticles. In order to prepare SiO2 nanoparticles dispersible into various types of organic solvents, anionic surfactant 1 was adsorbed on SiO2 nanoparticles and nanoparticles covered with a saturated amount of PEI in deionized water. The relationship among the additive content of anionic surfactant, adsorbed content of anionic surfactant, and the stability of the resulting nanoparticles in various solvents were investigated. Figure 6 shows the photographs of the PEI-adsorbed SiO2 suspension before and after the addition of the anionic surfactant in deionized water observed during the adsorption procedure and the transmittance value for each suspension. While the SiO2 suspension prior to the addition of the anionic surfactant was transparent, it gradually became cloudy when the anionic surfactant was added due to weak aggregation. The transmittance of the SiO2 suspension was reduced and it achieved its minimum value when the additive content was increased to 810 mg/g of SiO2. However, the transmittance value began to improve when the additive content was greater than 810 mg/g of SiO2 and finally the suspension became transparent when the additive content was greater than 1400 mg/g of SiO2. Figure 7a shows examples of TG curves for SiO2 modified with PEI and anionic surfactant 1. Compared with the curves for SiO2 modified only with PEI, nanoparticles modified with anionic surfactant 1 showed a larger weight loss, which demonstrates that anionic surfactant 1 has successfully adsorbed on PEI-modified SiO2 nanoparticles. Furthermore, the weight loss of SiO2 nanoparticles modified with PEI and the anionic surfactant increased 17946 DOI: 10.1021/la1030747

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Figure 7. (a) Examples of TG curves of SiO2 nanoparticles modified with PEI and surfactant 1. (b) Relationship between additive content, adsorbed content of surfactant 1, and ζ-potential of SiO2 nanoparticles at pH 7.8.

as the additive content of the anionic surfactant increased. The quantitative relationship between the additive content and the adsorbed content of anionic surfactant 1 is shown in Figure 7b. The adsorbed content of anionic surfactant linearly increases with increased amounts of the additive, which was different from the case of PEI adsorption on SiO2 nanoparticles shown in Figure 4. Note that it was quite difficult to characterize the adsorbed content of anionic surfactant 1 when the additive content was larger than 2000 mg/g of SiO2 due to difficulty in separating the SiO2 particles from the solution. The changes in ζ-potential caused by the addition of anionic surfactant are also shown Figure 7. It was found that the ζ-potential value linearly decreased as the additive content of the anionic surfactant increased up to 1400 mg/g of SiO2. However, the ζ-potential value was saturated when the additive anionic surfactant was greater than 1600 mg/g of SiO2. From these results, the adsorption structure of the anionic surfactant on PEI (that was adsorbed on the SiO2 nanoparticles) can be discussed. We hypothesize the weak aggregation of the SiO2 suspension after the addition of the anionic surfactant (Figure 6) is due to the adsorption of an anionic surfactant with an organic chain, which does not have a high affinity for water. When the anionic surfactant was added to the dispersed SiO2/PEI system, the cationic PEI adsorbed the anionic surfactant, which resulted in a decrease in the ζ-potential value and the stability of the suspension. The stability of the suspension (i.e., the transmittance of suspension) was worse when the ζ-potential value was near 0 mV and the additive anionic surfactant was 810 mg/g of SiO2. The anionic surfactant was fully adsorbed by the PEI under these conditions. Next, when the additive content of the anionic surfactant was increased to an amount greater than 810 mg/ of SiO2, the ζ-potential was negative and the suspension stability improved. Furthermore, the suspension was transparent when the additive anionic surfactant was more than 1600 mg/g of SiO2 and the ζ-potential value was saturated at ca. 20 mV. Under these Langmuir 2010, 26(23), 17943–17948

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Figure 8. Particle size distributions of (a) raw SiO2 nanoparticles and SiO2 nanoparticles modified with PEI and various amounts of anionic surfactant 1. The additive content of anionic surfactant 1 was (b) 810, (c) 990, and (d) 1400 mg/g of SiO2.

Figure 10. FT-IR-ATR spectra of Ag nanoparticles before and after surface modification. Ag nanoparticles capped with (a) oleylamine, (b) PEI, and (c) PEI/anionic surfactant.

Figure 11. Particle size distributions of Ag nanoparticles capped Figure 9. Average aggregated size of SiO2 nanoparticles modified by PEI and 810 mg/g of SiO2 of anionic surfactant.

conditions, a macromolecular structure such as a bilayer structure is expected to have formed on the SiO2 nanoparticles. As the bilayer structure formed of on the particle surface, the hydrophilic phosphate groups faced the solvent so that a stable aqueous suspension was obtained. To analyze the effect of the additive content of anionic surfactant on the redispersion stability of the resulting SiO2 nanoparticles in organic solvents, the SiO2 nanoparticles collected after anionic surfactant adsorption were redispersed into toluene and analyzed by the DLS method (Figure 8). When the additive content of the anionic surfactant was below 600 mg/g of SiO2, the collected SiO2 nanoparticles rapidly formed large aggregates and were not able to be redispersed into toluene and analyzed by the DLS method. It is expected that the nanoparticles were not stable in toluene since the PEI on the particle was not fully covered with anionic surfactant. In contrast, when the additive content of the anionic surfactant was between 810-1400 mg/g of SiO2, the resulting SiO2 nanoparticles were able to be redispersed into toluene at their primary particle size due to the coverage of the anionic surfactant. In order to analyze the dispersion stability of SiO2 nanoparticles fully covered with anionic surfactant in various organic solvents, their average aggregate size in various solvents was measured by the DLS method and are shown in Figure 9. SiO2 nanoparticles modified by PEI and the anionic surfactant can be redispersed into various organic solvents including alcohols, ketones, acetates, nitriles, and ethers. Slight differences in average aggregate sizes observed as a function of solvent type is expected to be due to the small differences in the affinity between organic chain of anionic surfactant and each solvent. Langmuir 2010, 26(23), 17943–17948

with (a) oleylamine (measured in toluene), (b) PEI (measured in deionized water), and (c) PEI/anionic surfactant (measured in toluene).

3.3. Preparation of Ag Nanoparticles and Their Dispersion into Various Organic Solvents. In order to ensure that the above-mentioned layer-by-layer surface modification procedure is applicable to other materials, Ag nanoparticles were modified by the PEI/anionic surfactant 1 and their dispersion stability in various solvents was investigated. For the surface modification of Ag nanoparticles, Ag nanoparticles coated with oleylamine were prepared. Then, the oleylamine capping agents were exchanged with PEI. Finally, the anionic surfactant was adsorbed on the PEI-modified Ag nanoparticles. Figure 10 shows the FT-IR (ATR) spectra of Ag nanoparticles before and after ligand exchange and the surface modification procedure. In Figure 10a, strong bands corresponding to νasCH3, νasCH2, νsCH2, νC-N, and δC-N can be observed at 2962, 2933, 2852, 1384, and 1635 cm-1, respectively, which confirms that the Ag nanoparticles were capped with oleylamine.29 In contrast, from Figure 10b, strong bands corresponding to oleylamine are reduced and new bands corresponding to δasNH3þ and δsNH3þ can be observed at 1524 and 1473 cm-1, respectively, which shows that oleylamine was successfully exchanged with PEI.30 When the anionic surfactant was adsorbed on PEI, peaks corresponding to νasCH3, νasCH2, νsCH2, CH2 wag, CH2 twist, νC-O-C stretch, and FCH2 rock appeared at 2962, 2933, 2852, 1350, 1280, 1108, and 969 cm-1, respectively.22 Figure 11 shows the dispersion stability of Ag nanoparticles capped with oleylamine, PEI, and PEI/anionic surfactant. (29) Lu, X.; Tuan, H.-Y.; Chen, J.; Li, Z.-Y.; Korgel, B. A.; Xia, Y. J. Am. Chem. Soc. 2007, 129, 1733. (30) Arkas, M.; Tsiourvas, D. J. Hazardous Mater. 2009, 170, 35.

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and the anionic surfactant can be redispersed into various organic solvents including alcohols, ketones, acetates, nitriles, and ethers. The surface modification of functional nanoparticles by PEI and the anionic surfactant will be an effective route to obtain nanoparticles dispersible in various organic solvents.

4. Conclusion

Figure 12. Average particle sizes of Ag nanoparticles modified by PEI and anionic surfactant in various organic solvents.

While oleylamine-coated Ag nanoparticles were dispersible in toluene, PEI-coated Ag nanoparticles were not dispersible in toluene. However, these particles were able to be dispersed in polar solvents such as water. Furthermore, when anionic surfactant 1 was adsorbed on PEI, the Ag nanoparticles became dispersible in toluene again. It should also be noted that a particle size of ca. 10 nm was maintained for each condition, as shown in Figure 11, which confirms that the Ag nanoparticles did not aggregate during the surface modification procedure. The FT-IR and DLS analyses demonstrate that ligand exchange of oleylamine to PEI and the adsorption of the anionic surfactant successfully occurred. To ensure that the Ag nanoparticles modified by PEI and anionic surfactant 1 can be redispersed into various organic solvents, their stability in various organic solvents were analyzed by the DLS method. As shown in Figure 12, Ag nanoparticles modified by PEI

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A layer-by-layer surface modification process was applied to SiO2 nanoparticles using cationic PEI and a branched anionic surfactant to achieve their complete dispersion in various organic solvents. The relationship among the additive content, adsorbed content, and stability of modified particles were studied during the adsorption of PEI and an anionic surfactant on SiO2 nanoparticles. In terms of PEI adsorption on SiO2 nanoparticles, the saturated adsorption of PEI on SiO2 nanoparticles played an important role in generating nanoparticles that redispersed in aqueous solvents. For the adsorption of anionic surfactant on PEI modified SiO2 nanoparticles, it was found that there was a possibility of formation of macromolecule structure such as bilayers by anionic surfactants when the additive content was large. SiO2 nanoparticles dispersible in various solvents were successfully prepared when the SiO2 nanoparticles were coated with a saturating amount of PEI, and then adsorbed with a monolayer of anionic surfactant. Furthermore, this layer-by-layer surface modification process can be applied to other materials such as Ag nanoparticles. Acknowledgment. This work was supported by a Grantin-Aid for Research Activity Start-up (No.21860026) from the Japan Society for the Promotion of Science.

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